This patch enables default_idle when power_save=off kernel bootoption is specified.

Earlier, this was done by setting ppc_md.power_save = NULL and henceHMT_low() and HMT_very_low() was called. Now this is defined underdefault_idle() and hence by setting boot_option_idle_override = 1,the cpuidle registration stuff does not happen and hence default_idleis chosen in cpuidle_idle_call.